The time capsule is a metal box and it wil contain 100 objects. It will probably be stored in an attic. The circuit to be triggered after 100 years will be an audio circuit (maybe with a surface transducer sticked to the metal of the case).

No doubt the power source is the stickiest problem here, but I'd also design some redundancy into the electronics. Source all parts based on reliability. That probably puts the flaps down on those cheap capacitors from eBay

Perhaps some sort of electromagnetic energy harvesting approach? Assuming an urban setting, the energy available might even increase over time. And if there is none in 100 years, it's probably because there's no one left to open the time capsule anyway.

A fuel cell could periodically charge a rechargeable battery or supercapacitor , therfore neutralising each other's shortcomings. I have a feeling you will end up needing a lot of power. Suppose the rechargeble could stay charged for 10 years, then the fuel cell should be able to work 11 times. This seems doable to me. Or you could use a wet battery and just replace the electrolyte with fresh one periodically.

Maybe temperature gradient exploitation using peltier devices?

I was going to suggest a radioisotope generator, but I doubt you can get one.

Or , someone mentioned a turtle. Don't they live 100 years? How much food could a turtle possibly eat in 100 years?

If there is no one to open the box in a hundred years, Who cares... But a water activated battery and some stored water and some instructions to "Pour Water Here to Activate" should work... All sealed in an inert gas? Nitrogen?, Neon, Argon to avoid corrosion. If the water contained a mild acid, Zinc and copper would serve well and I'm sure that there are better methods than what 2 minutes consideration gave me.

Bob

--> WA7EMS "The solution of every problem is another problem." -Johann Wolfgang von GoetheI do answer technical questions PM'd to me with whatever is in my clipboard

Most terrestrial batteries suffer from degradation from not being used as well as wear from use.

One of the longest known standalone power supplies would be the Radioisotope Thermoelectric Generators (RTGs) used in the Voyager spacecraft. Thirty-four years after launch, they are providing 57% of their rated output. Based on that, they will most not likely reach one century of operation.

There is the possibility of coupling a mechanical drive system to some form of generation. In the case of a time capsule, if it was suitably located hydroelectric generation would be a possibility. Depending on the environment for storage, a thermocouple might be usable to supplement the primary power.

In order to reach the goal of a century, a mechanical watch mechanism might be a better bet for the timing - if you can find a power source with sufficient shelf life to wait to be activated.

No matter which method you use to power it, you will have to take a note from spacecraft design, and make the entire system redundant, such that a failure of 50% of the components would still leave it in a usable state.

Seriously, though. It's going in an attic? Has anyone owned an attic for 100 years? Maybe I'm just American, but after 60 years or so, houses tend to get remodeled and/or demolished.

Seriously, we have cowsheds older than America The pier I walked on this afternoon was built in 1867, it looks pretty much the same as when it was built. And the pub I visited after has been a pub since late 1700's, it was however remodelled then as it was formally a place where cider was made. Not everything is pulled down every 60 years. So I reckon, a European loft has a good chance of surviving another 100 years (we gotta keep our chickens somewhere!)